Some U.S. officials continue to believe Pakistan’s Inter-Services Intelligence (ISI) agency poisoned the top CIA operative who presided over the secret raid that killed Osama bin Laden in Abbottabad, revealed The Washington Post.

Current and former U.S. officials told the Post that the now-retired Mark Kelton, 59, “was so violently ill that he was often doubled over in pain.”

Officials indicated that “trips out of the country for treatment proved futile. And the cause of his ailment was so mysterious … that both he and the agency began to suspect that he had been poisoned.”

Kelton was serving as the CIA station chief on May 2, 2011, when U.S. Navy SEAL Team Six killed the mastermind of the September 11 terrorist attacks, Osama bin Laden.

“His health has recovered after he had abdominal surgery,” reported the Post. “But agency officials continue to think that it is plausible — if not provable — that Kelton’s sudden illness was somehow orchestrated by Pakistan’s Inter-Services Intelligence agency, known as the ISI.”

Ahmed Shuja Pasha, the ISI chief during Kelton’s tenure in Pakistan, often refused to speak with the CIA chief, officials told the Post.

Pasha would even refuse to “utter his name, referring to the dour CIA station chief as ‘the cadaver,’” the officials added.

Pakistan has denied the allegations against the ISI.

Moreover, the Post reported, “Some of Kelton’s colleagues, including several who were based in Pakistan, remain skeptical that the ISI would risk Pakistan’s multibillion-dollar dependency on the United States by poisoning a high-ranking U.S. official.”

“Instead, skeptics believe that Kelton’s Moscow mind-set saw conspiracy in a condition more likely caused by bad food or the pressure of the job,” it added.

Kelton spent much of his career serving in Cold War outposts, including Moscow.

The counterterrorism partnership between the United States and nuclear-armed Pakistan has often been strained. U.S.-Pakistan relations were aggravated after the U.S. military killed al-Qaeda leader bin Laden without coordinating with Pakistani officials.

While the U.S. has continued to provide counterterrorism assistance to Pakistan, the Pentagon has repeatedly accused the country of serving as a sanctuary for terrorist groups that are actively fighting U.S.-led troops in neighboring Afghanistan.

U.S. officials reportedly conceded that the CIA never came across evidence showing that Kelton was indeed poisoned. Neither was Pakistan ever confronted with that allegation.

Privacy considerations “limit what we can say about any individual cases … but we have uncovered no evidence that Pakistani authorities poisoned a U.S. official serving in Pakistan,” acknowledged Dean Boyd, a spokesman for the CIA.

Nevertheless, current and former U.S. intelligence officials reportedly indicated that “the ISI has been linked to numerous plots against journalists, diplomats and other perceived adversaries and that the spy agency’s animosity toward Kelton was intense.”

Although the Post reported that it briefly engages with Kelton by phone, it added that the retired CIA officer declined multiple requests for an official interview.

The cause of his illness “was never clarified,” he stated, adding that the suspicions that the Pakistani spy agency had poisoned him “didn’t originate with” him.

“I’d rather let that whole sad episode lie,” he said, declining to answer questions about his illness or his tenure in Pakistan. “I’m very, very proud of the people I worked with who did amazing things for their country at a very difficult time. When the true story is told, the country will be very proud of them.”